In earth drilling, for example the drilling of wells for extraction of water, oil, natural gas, or coal bed methane, and in drilling for other purposes such as mineral exploration, mining, access to geothermal energy, or the formation of holes for building foundations, etc., it is desirable for the drill pipe loading and unloading equipment used in conjunction with a drilling mast to be capable of handling drill pipe in a range of diameters. There are several reasons. First, the ability to accommodate a range of pipe diameters gives the drill pipe loading and unloading apparatus improved versatility. That is, it can handle pipe used to make up a small diameter drill string as well as pipe used to make up a large diameter drill string. Second, the loading and unloading apparatus can also be used in drilling boreholes where a borehole is first drilled using small bit and a narrow drill string and later expanded using a larger bit on a larger diameter drill string. Third, if well casing is to be installed, it is convenient to supply the well casing to the drilling mast using the same loading and unloading apparatus as was used to supply the drill pipe.
It has been common practice to use mechanical pipe gripping devices in these loading and unloading apparatuses to move lengths of drill pipe into position for attachment to the top head of a drilling mast. However, most such gripping devices have been capable of handling only a very limited range of drill pipe diameters. The use of replacement jaws has been proposed to enable a gripping apparatus to accommodate different pipe sizes. However, to achieve this objective, it is necessary to keep a supply of gripping jaws of various sizes on hand.